1. Field of the Invention
The present invention relates to a memory material and a memory device including the memory material, and more particularly, to a phase change material containing carbon (C), a memory device including the phase change material, and a method of operating the memory device.
2. Description of the Related Art
A phase change random access memory (PRAM) is a non-volatile memory device and has advantages of a dynamic random access memory (DRAM) that is a representative volatile memory device.
A characteristic of a PRAM is that a storage node in which data is stored includes a phase change layer that is changed between an amorphous state and a crystalline state in accordance with temperature variations.
A commonly-used phase change layer of a PRAM is a Ge2Sb2Te5 layer (hereinafter, referred to as a GST layer). A GST layer has a low crystallization temperature (Tc) of 160° C. and thus a set current (Iset) is low.
Due to the fact that the distance between adjacent unit memory cells of a PRAM is getting smaller as the integration density of memory devices is increasing, if the Tc is low, when a selected cell is crystallized, a neighboring cell may be affected. That is, cell disturbance may occur. Cell disturbance may damage data written in an unselected cell and thus may deteriorate reliability of data. Also, if the Tc of the phase change layer is low, a retention characteristic or an infrared (IR) reflow characteristic may be poor.
Meanwhile, a melting temperature (Tm) of the GST layer at which the GST layer is changed from the crystalline state to the amorphous state is a relatively high temperature of 620° C. Thus, if the PRAM uses the GST layer as the phase change layer, a current for changing the phase change layer from the crystalline state to the amorphous state, in other words, a current for writing data, that is, a reset current (Ireset), may be relatively high.
In order to overcome the above disadvantage of the GST layer, an In3Sb1Te2 layer may be used as the phase change layer. In comparison to a GST layer, an In3Sb1Te2 layer has advantages as described below.
First, an In3Sb1Te2 layer has a Tc of 265° C. that is more than 100° C. higher than the Tc of a GST layer and thus thermal stability and retention characteristics of a PRAM including an In3Sb1Te2 layer as the phase change layer may be improved in comparison to a case when a GST layer is used.
Second, an In3Sb1Te2 layer has a relatively low Tm of 560° C. and thus the Ireset may be lowered.
Third, an In3Sb1Te2 layer has a high resistance in an amorphous state and a low resistance in a crystalline state and thus a sensing margin may be increased.
Fourth, a crystal structure of the In3Sb1Te2 layer is face-centered cubic (FCC) such that the phase change rate is fast. Thus, operating speed of a PRAM including the In3Sb1Te2 layer may be increased.
In spite of these advantages, the In3Sb1Te2 layer may be easily resolved into (InTe)2+(InSb)1 by a peritectic reaction. Thus, a single phase may not be easily formed and even if a single phase is formed, phase separation may easily occur.
As a result, in spite of its many advantages as the phase change layer of a PRAM, a conventional In3Sb1Te2 layer may not be easily applied to a PRAM.